A conventional safety window blind is designed to have a pulling cord looped on and rotating a rotary member for rolling a curtain. When an excess downward pulling force is applied to the pulling cord in case a child is accidentally entangled by the cord, the rotary member is drawn downward and the pulling cord is disengaged therefrom to quickly release the child. Referring to FIGS. 1 and 2, a safety control device as disclosed in WO 2015/174827 includes a housing 91, a roller 92 disposed within the housing 91, a slider 93 disposed between a rear side wall of the housing 91 and the roller 92 and biased forwardly by a spring 94 to urge the roller 92 forwardly to press against a support member 95, and a pulling cord 96 looped on the roller 92 to be pulled to rotate the roller 92 so as to transmit a torque to rotate a shaft 97 through the support member 95 for rolling a curtain. Once an excess downward pulling force is applied to the cord 96, the roller 92 is drawn out of the housing 91 and the cord 96 is disengaged from the roller 92 so as to prevent children from accidental strangulation.
Since the roller 92 is supported resiliently by the support member 95 and the slider 93 at front and rear sides with the resilient force of the spring 94, and since both the support member 95 and the slider 93 are movable in a front-and-rear direction relative to the housing 91, a tolerance connection between the housing 91 and each of the support member 95 and the slider 93 is required, which results in unsteady engagement between the roller 92 and each of the support member 95 and the slider 93, thus adversely affecting the transmission of the torque and the control of the roller 92.
Other safety control apparatus for a window blind, such as that disclosed in EP 1319793, U.S. Pat. No. 8,336,598, US 2011/0048657 or US 2012/0017399, suffers from the aforesaid drawback as well.